1. Field of the Invention
This invention relates to sliding contact-making structures constituted by a sleeveless cylinder and a piston of an internal combustion engine installed in an automobile, motorbike and the like, in particular, to such sliding contact-making structures allowing for preventing a plating coating deposited on the inside surface of the cylinder from peeling off, and preventing abrasion of both the plating coating and the skirt area of the piston. This invention also relates to a method for producing the above structure.
2. Background of the Art
A cylinder of a combustion engine normally comprises a cylinder unit and a piston which is fitted into the cylinder unit so as to freely slide in contact with the inside surface of the cylinder unit. In the above structure, the piston reciprocally slides upon the inside surface of the cylinder unit at a high speed when operated. As a result, frictional wear or seize tends to occur between the inside surface of the cylinder unit and the outer surface of the piston. In order to prevent such a problem, a cylinder unit wherein a plating coating with a high degree of hardness is formed on the inside surface is hitherto known. However, mere formation of a plating coating on the inside surface of the cylinder unit is not sufficient, and the following drawbacks may be associated with the plating coating.
First, near the end of the inside surface of the above cylinder unit on the side of a crankshaft, there is a step-like edge formed by the plating coating where it ends abruptly. Since there is play between the outermost surface of a piston skirt and the inside surface of the cylinder unit to allow the piston to smoothly slide upon the inside surface of the cylinder, the lower edge of the piston skirt reciprocally touches the very end or an adjacent area near the end of the plating coating, i.e., the step-like edge. As a result, the stress or impact caused by the above action of the piston when operated tends to be focused at the step-like edge of the plating coating, and the plating coating is prone to peel off from the step-like edge. Consequently, the life of the cylinder will be shortened.
Secondly, a piston normally comprises a piston body and at least one piston ring which is circumferentially installed at the outer surface of the piston body so as to contact the inside surface of the cylinder when sliding thereon. Airtightness of a combustion chamber is assured mainly at the outer surface of the piston ring in sliding contact. For this reason, matching the hardness of the inside surface of the cylinder (the plating coating), the piston ring has a sufficient hardness resistant to friction wear relative to the piston body in order to prevent friction wear occurring on the surface in sliding contact. However, not only the piston ring but also the piston body inevitably touches the inside surface of the cylinder when sliding, despite the fact that there is play between the piston body and the inside surface of the cylinder, resulting in friction wear unnecessarily occurring on the piston body itself, .especially the skirt area. Consequently, the life of the cylinder will be shortened.